Triac conduction angle control is a popular known technique for supplying an a.c. load with varying supply voltage. To carry out this control, the conduction angle of a triac device is adjusted by changing the switching instant of the triac device. In this way the conduction angle can be varied from 180° to 0°. The voltage r.m.s. value is a function of the conduction angle. This method frequently represents a cost-effective solution, and it is the most used technique for low-cost appliances, widely used in present day consumer products. A typical application of triac conduction angle control of an a.c. motor is depicted in FIG. 1A.
However, conduction angle control is not preferred for the latest designs because of its high harmonic pollution, which is not in compliance with strict European regulations.
There are available triac switching techniques which decrease the harmonic content of the supply current waveform. However, these techniques are not able to achieve significant effect which would be needed to comply with modern EMI/EMC (electromagnetic interference/electromagnetic compatibility) regulations.
Conduction angle control also produces low motor efficiency when supplied with non-sinusoidal current. Conduction angle control also suffers from unpleasant acoustic noise produced by motors supplied by means of triac devices.
Converter topology is another known technique for supplying single-phase a.c. loads. In this technique, the a.c. line voltage is converted to a d.c. voltage, usually using a diode bridge rectifier. The d.c. voltage is filtered by a filter capacitor and converted back to a.c. voltage by an inverter. The inverter is usually implemented as a single-phase bridge and the output voltage is determined by the switching of the bridge switches. Mainly PWM switching techniques are used. In this way it is possible to control both the amplitude and the frequency of the output voltage independently. This method is suitable for high power and high efficiency drives. A typical application of converter topology for controlling a single phase a.c. induction motor is depicted in FIG. 1B.
Converter topology suffers from the disadvantage that the components used for system realization are typically of high cost. Thus this topology is not suitable for low-cost applications.
Converter topology also suffers from the disadvantage of high harmonic content of the supply current waveform. Standard topologies use a diode bridge rectifier at the input. When the diode bridge is connected in parallel to the d.c. link filter capacitor, the current drawn from the a.c. line is non-sinusoidal with high peaks. To eliminate this some power factor correction technique needs to be implemented, thus increasing the system cost.
From U.S. Pat. No. 6,256,211 there is known a circuit device for driving an a.c. electric load incorporating a rectifying bridge that has a first input connected to one terminal of the electric load and a second input connected to the outlet of an a.c. mains supply. The rectifying bridge has output terminals connected to a power switch, which is controlled by an electric signal. The circuit device has two circuit loop-back links connected in parallel to the electric load. The first and second links are alternately activated by the positive and negative half-waves of the mains supply when the switch is in “off” state. FIG. 1C depicts this circuit device.
However, this approach has the disadvantage of high cos φ limitation. The device is capable of driving only electric loads whose power factor is close to one. If the phase shift between load current and supply voltage is higher, the device cannot ensure the sinusoidal load current, thus increasing the current harmonic content and lowering the electric device efficiency. Such a device is not capable of driving all possible types of a.c. loads (e.g., shaded pole a.c. induction motors, pure induction load, etc.). This significantly limits functionality and the circuit device cannot be used for driving a general a.c. load.
This approach also uses high number of power components, which increase power losses. This decreases the overall efficiency of the system, and also increases the device cost.
A need therefore exists for a circuit for supplying an electrical a.c. load wherein the abovementioned disadvantage(s) may be alleviated.